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Chemistry of Food
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OVERVIEW
Participants will learn about starches, proteins, vitamin C, and glucose, based on tests done with foods commonly
found in their kitchens, along with Benedict’s solution, indophenol, Biuret Solution, and Iodine.
CONCEPTS
Vitamin C is a vital nutrient for humans that aids our immune system and helps prevent disease. Many animals can make their
own vitamin C, but humans must get it from their diet, which is why the vitamin C content of the food we eat is important. At
one time a disease called scurvy was common among sailors, because they had no access to fruits and vegetables at sea. Scurvy is
caused by a lack of vitamin C.
Indophenol is an indicator that turns colorless in the presence of vitamin C. The fewer drops of juice you need for the color
change, the higher the vitamin C content in the juice.
Glucose is a simple sugar (carbohydrate) that plants produce by the process of photosynthesis. It is the primary source of energy
for our body's cells, and is able to enter our bloodstream quickly to provide energy right away. Without glucose, our bodies
wouldn't function. Plants store extra glucose in a more complex carbohydrate called starch. During digestion, our bodies break
the starch back down into glucose for our cells to use as an energy source.
Benedict’s Reagent is a solution of sodium citrate, sodium carbonate, and copper sulfate that changes from blue to red in the
presence of glucose.
Starches are odorless tasteless white substances occurring widely in plant tissue, and are obtained chiefly from cereals and
potatoes. They are a polysaccharide that function as a carbohydrate store, and are an important part of the human diet.
Iodine is used to test for the presence of starch. It is dissolved in an aqueous solution of potassium iodide, and reacts in the
presence of a starch, producing a purple-black color.
Lipids are fats and oils, made of molecules that don't dissolve in water. They are very important for our body functions because
they transport vitamins, help form cell walls, and store energy long-term. Eating too much fat can be very unhealthy, but every
good diet will contain a moderate amount.
Biuret Reagent is a solution of biuret (allophanamide) treated with cupric sulfate and sodium hydroxide. In the presence of
protein, this blue solution will change color to pink-purple. It is used to determine the presence of peptide bonds in proteins.
Proteins are large biological molecules, or macromolecules, consisting of one or more long chains of amino acids.
OBJECTIVES
After this lesson, students will be able to identify starches, proteins, vitamin C, and glucose in their everyday diets.
MATERIALS
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Peanut Butter
Lemon Juice
Orange Juice
Apple Juice
Saltine Crackers
Yogurt
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Egg Whites
Milk
Water
Biuret’s Reagent
Indophenol
Benedict’s Reagent
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Iodine
Test Tubes
Beakers
Hot Plate
Brown Paper Bags
Markers
PROCEDURES
Activity 1 Stop the Scurvy
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Choose several different juices to test for Vitamin C- Apple, Orange, and Lemon is what we are using.
Make some indophenol solution by combining a small amount (less than 1/8 teaspoon) with 1 cup of water. Stir until it
is well mixed.
Put 15 drops of indophenol solution into a test tube.
Add one of the juices you chose to the indophenol drop by drop. Record how many drops it takes to turn the blue
indophenol colorless.
Repeat with the other juices and compare the number of drops of each that you added
Indophenol is an indicator that turns colorless in the presence of vitamin C. The fewer drops of juice you need for the color
change, the higher the vitamin C content in the juice. Which fruits had the most vitamin C?
Activity 2 Sugar & Starch-Glucose Test
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Chew a saltine, but do not swallow! The saliva is needed to test for the presence of glucose.
Fill two test tubes with an inch of Benedict's solution each. To one, add 15 drops of the saltine-and-water solution, to
the other, add the saltine-and-saliva solution. (Use two different pipets and label the test tubes so you can tell them
apart.)
Cover your test tubes with parafilm, and then set them in the beaker of hot water.
Wait 3 minutes.
Remove the test tubes from the hot water and allow them to cool. Swirl the contents and observe the color of the
liquid. Is there a difference between the two?
Without glucose, our bodies wouldn't function. Plants store extra glucose in a more complex carbohydrate called starch. During
digestion, our bodies break the starch back down into glucose for our cells to use as an energy source.
In this project you saw that process in action. Saltine crackers have lots of starch. When you chewed one of the crackers, an
enzyme in your saliva, called amylase, started to break the starch down into glucose. The color difference in your two test tubes
proves that the chewed-up cracker contained glucose while the other didn't.
Activity 3 Looking for Lipids-Fat Test
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Cut off the side of a brown paper bag so you have a sheet of brown paper.
Make solutions of several different foods you want to test for lipids. We are using milk, peanut butter, yogurt and
water.
Mix ½ teaspoon of ground food with 1 teaspoon water.
Use a permanent marker to divide the brown paper into several sections and label each section with the name of the
food.
Put three drops of each food solution on the paper in its section. Use a clean pipet for each test.
We are testing water in one section to show the result with a substance that doesn't have any lipids.
Wait a few minutes until the liquid has evaporated. Hold up the paper to the light and look at each spot. The foods that
contain lipids will leave a greasy mark that turns the brown paper translucent. Which foods have lipids? Are some of
the greasy spots bigger than others?
Activity 4 Testing for Proteins
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Students will be given 3 test tubes, one with an egg white mixture, one with a peanut butter mixture, and one with a
yogurt mixture. They will not know what is in each test tube.
½ tsp food to 1 tsp water.
Add 3 drops of Biuret’s Reagent to each test tube.
Color change?
Blue to violet or blue to pink indicates the presence of protein in the solution.
Try to guess what each solution is, based on the color change with the Biuret’s.
References
Home Science Tools
www.hometrainingtools.com